Description (applicant's description): Gene-encoded peptide antimicrobials are ubiquitous components of host defenses in animals, including humans. They are present in epithelial surfaces and phagocytic cells, and play an important role in the initial phases of resistance to microbial invasion. These peptides are typically 20 to 40 amino acids in length, with a folded size approximating the membrane thickness. Unlike the conventional antibiotics which target protein receptors, the antimicrobial peptides act on cell's plasma membrane. While a protein has a definitive structure for recognition, a membrane is a two-dimensional molecular fluid. Thus the question arises: how do the antimicrobials distinguish species self from infectious nonself? Moreover, different antimicrobials preferentially kill different pathogens and some exhibit varying levels of lytic activity against different mammalian cells. Our goal is to elucidate the mechanism of antimicrobial peptides by studying the supramolecular structure and the energetic property of lipid bilayers containing peptides. We found that the peptides can bind to lipid bilayers in two different ways. In one (S) state, the peptides are adsorbed in the polar region of the bilayer. In another (I) state, the peptides insert transmembrane and form multiple pores, a lethal condition if occurs in cell membranes. Our hypothesis is that the condition for the transition from the S to the I state depends on the lipid composition and the chemical condition of the cell membrane, and this might explain the cell type selectivity exhibited by the antimicrobials. We will perform new experiments to investigate this hypothesis. We will conduct X-ray diffraction of the I state in both the fluid and crystalline phases with peptides labeled by heavy atoms to delineate the pore structures. We will try to describe the peptide-membrane interactions in terms of energetics. An unusual term in such interactions is the energy of bilayer deformation. We will measure the response functions of lipid bilayers to compute this energy. We will also use a new technique of inelastic X-ray scattering to study the collective dynamics of lipid molecules that might shed light on how molecules enter lipid bilayers. Finally, we will extend our study to include peptide interactions with key structural elements of bacterial membranes.